Making of lead arsenates



Oct. 20, 1936. o s I 2,057,985

MAKING OF LEAD ARSENATES Filed Feb. 24, 1935 41/ lzer Zia Ur' Ufinl? 4am;

IN V EN TOR.

A TTO IVEYS.

Patented Oct. 20, 1936 UNITED STATES PATENT orrica signor to Thomson Chemical Corporation, a corporation, of California Application February 24, 1933, Serial No. 658,448 3 Claims. (01. 23--54) The manufacture of lead arsenate is to-day confined almost exclusively to the interaction between litharge and arsenic acid, as this is ordinarily a cheaper procedure than preparing it by the precipitation of some soluble salt of lead by sodium arsenate. Y

However, this manufacture is also accompanied by certain'disadvantages in the finished product, which seem inherent in' the process of manufacture. Chief of these is the fact that if we regard the most highly arseniated compound with 33% of A5205, i. e. PbHASO4, as the type under discussion, we find that this substance undergoes prolonged hydrolysis ifsubjeoted to the action of continuously renewed water. A reasonable amount of arsenic acid may thus be extracted, and the remaining compound will be found to havebecome somewhat enriched in lead.

This hydrolysis is one of the greatest, if not thesolecause, of much of the damage done to foliage, which at certain seasons, or under certain climatic conditions, is likely to'follow the use of the lead arsenates that now dominate the'market. It is my chief object, therefore, to make, an arsenate which is far more stable than those commonly met with, and which has nosuch action. .f An easy test to show the'distinction between these litharge-arsenic acid derivatives and such a one as'I am about to describe, is to place a gram sample of each in 500 cubic centimeters of distilled water and shake thoroughly every hour for 24 hours, meanwhilemaintaining the containers at a temperature of 32 centigrade by means of a water bath. To-the clear, filtered solution from each flask now, add a drop. of silver nitratesolution. It is to be noted, then, that a decided brown precipitate of silver arsenate will at once appear in the case of the sample made from litharge and arsenic acid, while the sample made in the manner as hereinafter indicated will show, at the most, a faint whitish opalescence (due to traces of arsenite) "orelse remain entirely clear.

The chief disadvantage to precipitated arsenate, in contradistinction to that made directly from litharge, is its greater cost. I have overcome this handicap to a certain extent by drawing upon a cheap source of lead eminently suited for this purpose, and in a co-pending patent application, Ser. No. 609,765, I have indicated how this may be made a source of soluble salts of lead of a high degree of purity. I will now described some manipulative features in the process of manufacture whereby additional economies are effected. The accompanying flow sheet diagrammatically illustrates the steps involved in my processand the chronological order of the addition of the various reagents.

As a soluble salt of lead I prefer the nitrate, not only because of its ready solubility, but because of the oxidizing effect of residual solutions of this salt upon any unoxidized arsem'te in the final product. I prefer-to link the manufacture of nitric acid from sulphuric acid and sodium nitrate with the manufacture of lead nitrate for several reasons: r

1. Instead of condensing the nitric acidin the customary manner I prefer toperrni t the fumes from the retort to condense directly in the suspension of crude litharge from which the nitrate is being manufactured, or, in the end solution from the final precipitation of arsenate which then is used once more, to make fresh nitrate of lead. In the latter case, a small coke tower, irrigated with the end solution, is interposed between the'retort and the dissolving tank for litharge. Two advantages are gained by this procedure; (a) the final solution which is principally nitrate of soda is thus economically concentrated before final evaporation; and, (b),'any lead nitrate or unusued arsenic or free nitricacid therein is ef: ficiently utilized.

2. I find that if I pass the fumes from the retort through a short condenser, and thus split off afraction as condensed strong nitric, that this type of acid is particularly well suited for oxidation of White arsenic into arsenic acid. The balance of the nitric acid is then condensed asbefore. In this manner there is no handling of strong'nitric acid at all, save that part used on the arsenic, and all expense for plant and mainteriance is eliminated. V

3. The oxidation of white arsenic by means of this nitric acid proceeds in the customary manner,

and is accompanied by' a strong evolution of V nitrous fumes. If these be now drawn intothe ,coketower, before referred to, they will be con- Manifestly, these advantages can only be realized where the manufacture of nitric acid is linked with the production of both lead nitrate and lead arsenate,-the use of the end solution becoming the link which converts three otherwise unrelated types of endeavor into a single process for the production of lead arsenate at an extremely low price.

In the manufacture of the arsenate of lead, considered by itself, there are also many innovations. It is highly desirable that the ultimate particle or crystal of lead arsenate should not only be minute but should possess, in addition, a peculiar feathery structure, which gives great covering power as well as great power of adhesion after application.

The addition of even pure crystallized Na2HAsO4, or its basic substitute Na3AsO4, to a pure solution of lead nitrate cannot produce the effect at which I aim. Neither will a mixture of arsenic acid and lead nitrate, subsequently neutralized with soda ash or caustic soda, produce a desirable product. However correct these compounds may be from the chemical viewpoint, they will be greatly deficient in the physical characteristics which are equally essential if we are to obtain optimum effects in the application of the product to fruit, flower, or foliage.

My invention produces this desirable chemical and physical effect when the solutions of arsenic acid and lead nitrate, prepared in accordance with the description already given, be used in the following manner:

At the start, I add to the arsenic solution only a part of the soda ash ultimately needed to bring it into the condition required to obtain the correct proportion of lead and arsenic oxide in the finished arsenate of lead,in general a little less than three-fourths of the total. I then add an excess of the lead solution, and leave the ingredients to interact under efiicient agitation for some time. In this state, (that is, in a solution containing free nitric acid, free arsenic acid, lead nitrate, nitrous compounds, arsenate and arsenite of lead), very efficient oxidation takes place, and virtually all of the AS203 which may have escaped oxidation in the preliminary step before described is here converted to the higher form. In addition to this desirable chemical after-effect, I obtain by this procedure the very type of a crystal which is so essential to the covering power of the lead arsenate.

About half of the remaining soda is now added, and, when efiervescence has ceased, I add the rest of the required amount of lead nitrate. Care must be taken throughout that the solution is always kept acid to methyl orange, even if this involves the addition of fresh nitric acid. The last of the soda is now added, and the acidity of the end solution thus reduced to the pre-determined limit. Provision is made for this step because of the solubility of freshly precipitated lead arsenate in very dilute nitric acid, whereby much loss could be sustained. The magma is then heated and passed to filters and driers in the customary manner, and the product thus obtained is air floated and packed for the market.

The filtrate from the filtering operation constitutes the end solution already referred to several times. To the extent possible, it is cooled and used as the condensing-absorbing medium for the nitric acid operation as before indicated, or it may be added directly to the litharge dissolving tank in the event that strong nitric acid were made and condensed in the usual manner. The portion remaining of the end solution is neutralized with more soda, and the precipitate of arsenic and lead removed. The solution is then evaporated to dryness, or to crystallization, and added to the nitric acid retort, thus returning to the process, cyclically, substantially all the nitrate used.

Basic modifications containing less arsenic and more lead may be made by adding lead hydrate, in the amount needed to obtain the end desired, to the magma of PbI-IAsO4, prepared as before described.

Having thus described my invention, I claim:

1. The method of manufacturing lead arsenate consisting of commencing with a solution of white arsenic in nitric acid substantially saturated with nitrous fumes; then adding soda to neutralize the free acid present but stopping short when the solution is as yet more acid than corresponds to sodium di-arsenate; then adding a solution of lead nitrate, prepared so that it is substantially saturated with nitrous fumes, until an excess of lead is present; then making further additions of soda and lead nitrate alternately until the filtrate no longer tests for the presence of arsenic acid by the silver nitrate test, meanwhile taking precautions that the lead in the filtrate shall always be kept in excess and that the filtrate shall always be kept acid to methyl orange; then separating the precipitated lead arsenate from the end solution; and then washing said lead arsenate substantially free from soluble soda salts.

2. The method of manufacturing lead arsenate of a lower arsenic oxide content than 30% consisting of first preparing a batch of lead arsenate by the process recited in claim 1; then commingling same with a batch of freshly prepared lead hydrate in sufficient quantity to reduce the arsenic oxide content of the mixture to the predetermined percentage; and then washing said mixture substantially free from all soda salts.

3. A process for the manufacture of lead arsenate including the steps recited in claim 1 with the added step that the end solution of said reaction be fully neutralized, clarified, and evaporated for its constituent sodium nitrate, and that said nitrate thus obtained be then returned to the process as raw material for the manufacture of the requisite nitric acid.

ALFRED M. THOMSEN. 

